library new_gradient_app_bar;

import 'dart:math' as math;

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart';

const double _kLeadingWidth =
    kToolbarHeight; // So the leading button is square.

// Bottom justify the kToolbarHeight child which may overflow the top.
class _ToolbarContainerLayout extends SingleChildLayoutDelegate {
  const _ToolbarContainerLayout();

  @override
  BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
    return constraints.tighten(height: kToolbarHeight);
  }

  @override
  Size getSize(BoxConstraints constraints) {
    return Size(constraints.maxWidth, kToolbarHeight);
  }

  @override
  Offset getPositionForChild(Size size, Size childSize) {
    return Offset(0.0, size.height - childSize.height);
  }

  @override
  bool shouldRelayout(_ToolbarContainerLayout oldDelegate) => false;
}

// TODO(eseidel): Toolbar needs to change size based on orientation:
// https://material.io/design/components/app-bars-top.html#specs
// Mobile Landscape: 48dp
// Mobile Portrait: 56dp
// Tablet/Desktop: 64dp

/// A material design app bar.
///
/// An app bar consists of a toolbar and potentially other widgets, such as a
/// [TabBar] and a [FlexibleSpaceBar]. App bars typically expose one or more
/// common [actions] with [IconButton]s which are optionally followed by a
/// [PopupMenuButton] for less common operations (sometimes called the "overflow
/// menu").
///
/// App bars are typically used in the [Scaffold.appBar] property, which places
/// the app bar as a fixed-height widget at the top of the screen. For a scrollable
/// app bar, see [SliverNewGradientAppBar], which embeds an [NewGradientAppBar] in a sliver for use in
/// a [CustomScrollView].
///
/// When not used as [Scaffold.appBar], or when wrapped in a [Hero], place the app
/// bar in a [MediaQuery] to take care of the padding around the content of the
/// app bar if needed, as the padding will not be handled by [Scaffold].
///
/// The GradientAppBar displays the toolbar widgets, [leading], [title], and [actions],
/// above the [bottom] (if any). The [bottom] is usually used for a [TabBar]. If
/// a [flexibleSpace] widget is specified then it is stacked behind the toolbar
/// and the bottom widget. The following diagram shows where each of these slots
/// appears in the toolbar when the writing language is left-to-right (e.g.
/// English):
///
/// ![The leading widget is in the top left, the actions are in the top right,
/// the title is between them. The bottom is, naturally, at the bottom, and the
/// flexibleSpace is behind all of them.](https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.png)
///
/// If the [leading] widget is omitted, but the [NewGradientAppBar] is in a [Scaffold] with
/// a [Drawer], then a button will be inserted to open the drawer. Otherwise, if
/// the nearest [Navigator] has any previous routes, a [BackButton] is inserted
/// instead. This behavior can be turned off by setting the [automaticallyImplyLeading]
/// to false. In that case a null leading widget will result in the middle/title widget
/// stretching to start.
///
/// {@tool snippet --template=stateless_widget_material}
///
/// This sample shows an [NewGradientAppBar] with two simple actions. The first action
/// opens a [SnackBar], while the second action navigates to a new page.
///
/// ```dart preamble
/// final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
/// final SnackBar snackBar = const SnackBar(content: Text('Showing Snackbar'));
///
/// void openPage(BuildContext context) {
///   Navigator.push(context, MaterialPageRoute(
///     builder: (BuildContext context) {
///       return Scaffold(
///         appBar: GradientAppBar(
///           title: const Text('Next page'),
///         ),
///         body: const Center(
///           child: Text(
///             'This is the next page',
///             style: TextStyle(fontSize: 24),
///           ),
///         ),
///       );
///     },
///   ));
/// }
/// ```
///
/// ```dart
/// Widget build(BuildContext context) {
///   return Scaffold(
///     key: scaffoldKey,
///     appBar: GradientAppBar(
///       title: const Text('GradientAppBar Demo'),
///       actions: <Widget>[
///         IconButton(
///           icon: const Icon(Icons.add_alert),
///           tooltip: 'Show Snackbar',
///           onPressed: () {
///             scaffoldKey.currentState.showSnackBar(snackBar);
///           },
///         ),
///         IconButton(
///           icon: const Icon(Icons.navigate_next),
///           tooltip: 'Next page',
///           onPressed: () {
///             openPage(context);
///           },
///         ),
///       ],
///     ),
///     body: const Center(
///       child: Text(
///         'This is the home page',
///         style: TextStyle(fontSize: 24),
///       ),
///     ),
///   );
/// }
/// ```
/// {@end-tool}
///
/// See also:
///
///  * [Scaffold], which displays the [NewGradientAppBar] in its [Scaffold.appBar] slot.
///  * [SliverNewGradientAppBar], which uses [NewGradientAppBar] to provide a flexible app bar that
///    can be used in a [CustomScrollView].
///  * [TabBar], which is typically placed in the [bottom] slot of the [NewGradientAppBar]
///    if the screen has multiple pages arranged in tabs.
///  * [IconButton], which is used with [actions] to show buttons on the app bar.
///  * [PopupMenuButton], to show a popup menu on the app bar, via [actions].
///  * [FlexibleSpaceBar], which is used with [flexibleSpace] when the app bar
///    can expand and collapse.
///  * <https://material.io/design/components/app-bars-top.html>
class NewGradientAppBar extends StatefulWidget implements PreferredSizeWidget {
  /// Creates a material design app bar.
  ///
  /// The arguments [primary], [toolbarOpacity], [bottomOpacity]
  /// and [automaticallyImplyLeading] must not be null. Additionally, if
  /// [elevation] is specified, it must be non-negative.
  ///
  /// If [elevation], [brightness], [iconTheme],
  /// [actionsIconTheme], or [textTheme] are null, then their [GradientAppBarTheme]
  /// values will be used. If the corresponding [GradientAppBarTheme] property is null,
  /// then the default specified in the property's documentation will be used.
  ///
  /// Typically used in the [Scaffold.appBar] property.
  NewGradientAppBar({
    Key? key,
    this.leading,
    this.automaticallyImplyLeading = true,
    this.title,
    this.actions,
    this.flexibleSpace,
    this.bottom,
    this.elevation,
    this.shape,
    this.gradient,
    this.brightness,
    this.iconTheme,
    this.actionsIconTheme,
    this.textTheme,
    this.primary = true,
    this.centerTitle,
    this.titleSpacing = NavigationToolbar.kMiddleSpacing,
    this.toolbarOpacity = 1.0,
    this.bottomOpacity = 1.0,
  })  : assert(elevation == null || elevation >= 0.0),
        preferredSize = Size.fromHeight(
            kToolbarHeight + (bottom?.preferredSize.height ?? 0.0)),
        super(key: key);

  /// A widget to display before the [title].
  ///
  /// If this is null and [automaticallyImplyLeading] is set to true, the
  /// [NewGradientAppBar] will imply an appropriate widget. For example, if the [NewGradientAppBar] is
  /// in a [Scaffold] that also has a [Drawer], the [Scaffold] will fill this
  /// widget with an [IconButton] that opens the drawer (using [Icons.menu]). If
  /// there's no [Drawer] and the parent [Navigator] can go back, the [NewGradientAppBar]
  /// will use a [BackButton] that calls [Navigator.maybePop].
  ///
  /// {@tool sample}
  ///
  /// The following code shows how the drawer button could be manually specified
  /// instead of relying on [automaticallyImplyLeading]:
  ///
  /// ```dart
  /// GradientAppBar(
  ///   leading: Builder(
  ///     builder: (BuildContext context) {
  ///       return IconButton(
  ///         icon: const Icon(Icons.menu),
  ///         onPressed: () { Scaffold.of(context).openDrawer(); },
  ///         tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
  ///       );
  ///     },
  ///   ),
  /// )
  /// ```
  /// {@end-tool}
  ///
  /// The [Builder] is used in this example to ensure that the `context` refers
  /// to that part of the subtree. That way this code snippet can be used even
  /// inside the very code that is creating the [Scaffold] (in which case,
  /// without the [Builder], the `context` wouldn't be able to see the
  /// [Scaffold], since it would refer to an ancestor of that widget).
  ///
  /// See also:
  ///
  ///  * [Scaffold.appBar], in which an [NewGradientAppBar] is usually placed.
  ///  * [Scaffold.drawer], in which the [Drawer] is usually placed.
  final Widget? leading;

  /// Controls whether we should try to imply the leading widget if null.
  ///
  /// If true and [leading] is null, automatically try to deduce what the leading
  /// widget should be. If false and [leading] is null, leading space is given to [title].
  /// If leading widget is not null, this parameter has no effect.
  final bool automaticallyImplyLeading;

  /// The primary widget displayed in the appbar.
  ///
  /// Typically a [Text] widget containing a description of the current contents
  /// of the app.
  final Widget? title;

  /// Widgets to display after the [title] widget.
  ///
  /// Typically these widgets are [IconButton]s representing common operations.
  /// For less common operations, consider using a [PopupMenuButton] as the
  /// last action.
  final List<Widget>? actions;

  /// This widget is stacked behind the toolbar and the tab bar. It's height will
  /// be the same as the app bar's overall height.
  ///
  /// A flexible space isn't actually flexible unless the [NewGradientAppBar]'s container
  /// changes the [NewGradientAppBar]'s size. A [SliverNewGradientAppBar] in a [CustomScrollView]
  /// changes the [NewGradientAppBar]'s height when scrolled.
  ///
  /// Typically a [FlexibleSpaceBar]. See [FlexibleSpaceBar] for details.
  final Widget? flexibleSpace;

  /// This widget appears across the bottom of the app bar.
  ///
  /// Typically a [TabBar]. Only widgets that implement [PreferredSizeWidget] can
  /// be used at the bottom of an app bar.
  ///
  /// See also:
  ///
  ///  * [PreferredSize], which can be used to give an arbitrary widget a preferred size.
  final PreferredSizeWidget? bottom;

  /// The z-coordinate at which to place this app bar relative to its parent.
  ///
  /// This controls the size of the shadow below the app bar.
  ///
  /// The value is non-negative.
  ///
  /// If this property is null, then [ThemeData.appBarTheme.elevation] is used,
  /// if that is also null, the default value is 4, the appropriate elevation
  /// for app bars.
  final double? elevation;

  /// The material's shape as well its shadow.
  ///
  /// A shadow is only displayed if the [elevation] is greater than
  /// zero.
  final ShapeBorder? shape;

  /// The color to use for the app bar's material. Typically this should be set
  /// along with [brightness], [iconTheme], [textTheme].
  ///
  /// If this property is null, then [ThemeData.appBarTheme.color] is used,
  /// if that is also null, then [ThemeData.primaryColor] is used.
  final Gradient? gradient;

  /// The gradient displayed at the appbar.
  ///
  /// If this property is null, then [ThemeData.appBarTheme.brightness] is used,
  /// if that is also null, then [ThemeData.primaryColorBrightness] is used.
  final Brightness? brightness;

  /// The color, opacity, and size to use for app bar icons. Typically this
  /// is set along with [backgroundColor], [brightness], [textTheme].
  ///
  /// If this property is null, then [ThemeData.appBarTheme.iconTheme] is used,
  /// if that is also null, then [ThemeData.primaryIconTheme] is used.
  final IconThemeData? iconTheme;

  /// The color, opacity, and size to use for the icons that appear in the app
  /// bar's [actions]. This should only be used when the [actions] should be
  /// themed differently than the icon that appears in the app bar's [leading]
  /// widget.
  ///
  /// If this property is null, then [ThemeData.appBarTheme.actionsIconTheme] is
  /// used, if that is also null, then this falls back to [iconTheme].
  final IconThemeData? actionsIconTheme;

  /// The typographic styles to use for text in the app bar. Typically this is
  /// set along with [brightness] [backgroundColor], [iconTheme].
  ///
  /// If this property is null, then [ThemeData.appBarTheme.textTheme] is used,
  /// if that is also null, then [ThemeData.primaryTextTheme] is used.
  final TextTheme? textTheme;

  /// Whether this app bar is being displayed at the top of the screen.
  ///
  /// If true, the app bar's toolbar elements and [bottom] widget will be
  /// padded on top by the height of the system status bar. The layout
  /// of the [flexibleSpace] is not affected by the [primary] property.
  final bool primary;

  /// Whether the title should be centered.
  ///
  /// Defaults to being adapted to the current [TargetPlatform].
  final bool? centerTitle;

  /// The spacing around [title] content on the horizontal axis. This spacing is
  /// applied even if there is no [leading] content or [actions]. If you want
  /// [title] to take all the space available, set this value to 0.0.
  ///
  /// Defaults to [NavigationToolbar.kMiddleSpacing].
  final double titleSpacing;

  /// How opaque the toolbar part of the app bar is.
  ///
  /// A value of 1.0 is fully opaque, and a value of 0.0 is fully transparent.
  ///
  /// Typically, this value is not changed from its default value (1.0). It is
  /// used by [SliverNewGradientAppBar] to animate the opacity of the toolbar when the app
  /// bar is scrolled.
  final double toolbarOpacity;

  /// How opaque the bottom part of the app bar is.
  ///
  /// A value of 1.0 is fully opaque, and a value of 0.0 is fully transparent.
  ///
  /// Typically, this value is not changed from its default value (1.0). It is
  /// used by [SliverNewGradientAppBar] to animate the opacity of the toolbar when the app
  /// bar is scrolled.
  final double bottomOpacity;

  /// A size whose height is the sum of [kToolbarHeight] and the [bottom] widget's
  /// preferred height.
  ///
  /// [Scaffold] uses this this size to set its app bar's height.
  @override
  final Size preferredSize;

  bool? _getEffectiveCenterTitle(ThemeData themeData) {
    if (centerTitle != null) return centerTitle;
    assert(true);
    switch (themeData.platform) {
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
        return false;
      case TargetPlatform.iOS:
        return actions == null || actions!.length < 2;
      case TargetPlatform.linux:
        return false;
      case TargetPlatform.macOS:
        return false;
      case TargetPlatform.windows:
        return false;
    }
  }

  @override
  _NewGradientAppBarState createState() => _NewGradientAppBarState();
}

class _NewGradientAppBarState extends State<NewGradientAppBar> {
  static const double _defaultElevation = 4.0;

  void _handleDrawerButton() {
    Scaffold.of(context).openDrawer();
  }

  void _handleDrawerButtonEnd() {
    Scaffold.of(context).openEndDrawer();
  }

  @override
  Widget build(BuildContext context) {
    assert(!widget.primary || debugCheckHasMediaQuery(context));
    assert(debugCheckHasMaterialLocalizations(context));
    final ThemeData themeData = Theme.of(context);
    final AppBarTheme appBarTheme = AppBarTheme.of(context);
    final ScaffoldState scaffold = Scaffold.of(context);
    final ModalRoute<dynamic>? parentRoute = ModalRoute.of(context);

    final bool hasDrawer = scaffold.hasDrawer;
    final bool hasEndDrawer = scaffold.hasEndDrawer;
    final bool canPop = parentRoute?.canPop ?? false;
    final bool useCloseButton =
        parentRoute is PageRoute<dynamic> && parentRoute.fullscreenDialog;

    IconThemeData overallIconTheme =
        widget.iconTheme ?? appBarTheme.iconTheme ?? themeData.primaryIconTheme;
    IconThemeData actionsIconTheme = widget.actionsIconTheme ??
        appBarTheme.actionsIconTheme ??
        overallIconTheme;
    TextStyle centerStyle = (widget.textTheme?.headline6 ??
        appBarTheme.textTheme?.headline6 ??
        themeData.primaryTextTheme.headline6)!;
    TextStyle? sideStyle = widget.textTheme?.bodyText2 ??
        appBarTheme.textTheme?.bodyText2 ??
        themeData.primaryTextTheme.bodyText2;

    if (widget.toolbarOpacity != 1.0) {
      final double opacity =
          const Interval(0.25, 1.0, curve: Curves.fastOutSlowIn)
              .transform(widget.toolbarOpacity);
      if (centerStyle.color != null)
        centerStyle = centerStyle.copyWith(
            color: centerStyle.color!.withOpacity(opacity));
      if (sideStyle?.color != null)
        sideStyle =
            sideStyle!.copyWith(color: sideStyle.color!.withOpacity(opacity));
      overallIconTheme = overallIconTheme.copyWith(
          opacity: opacity * (overallIconTheme.opacity ?? 1.0));
      actionsIconTheme = actionsIconTheme.copyWith(
          opacity: opacity * (actionsIconTheme.opacity ?? 1.0));
    }

    Widget? leading = widget.leading;
    if (leading == null && widget.automaticallyImplyLeading) {
      if (hasDrawer) {
        leading = IconButton(
          icon: const Icon(Icons.menu),
          onPressed: _handleDrawerButton,
          tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
        );
      } else {
        if (canPop)
          leading = useCloseButton ? const CloseButton() : const BackButton();
      }
    }
    if (leading != null) {
      leading = ConstrainedBox(
        constraints: const BoxConstraints.tightFor(width: _kLeadingWidth),
        child: leading,
      );
    }

    Widget? title = widget.title;
    if (title != null) {
      bool? namesRoute;
      switch (defaultTargetPlatform) {
        case TargetPlatform.android:
        case TargetPlatform.fuchsia:
          namesRoute = true;
          break;
        case TargetPlatform.iOS:
        case TargetPlatform.linux:
        case TargetPlatform.macOS:
        case TargetPlatform.windows:
          break;
      }
      title = DefaultTextStyle(
        style: centerStyle,
        softWrap: false,
        overflow: TextOverflow.ellipsis,
        child: Semantics(
          namesRoute: namesRoute,
          child: title,
          header: true,
        ),
      );
    }

    Widget? actions;
    if (widget.actions != null && widget.actions!.isNotEmpty) {
      actions = Row(
        mainAxisSize: MainAxisSize.min,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: (widget.actions)!,
      );
    } else if (hasEndDrawer) {
      actions = IconButton(
        icon: const Icon(Icons.menu),
        onPressed: _handleDrawerButtonEnd,
        tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
      );
    }

    // Allow the trailing actions to have their own theme if necessary.
    if (actions != null) {
      actions = IconTheme.merge(
        data: actionsIconTheme,
        child: actions,
      );
    }

    final Widget toolbar = NavigationToolbar(
      leading: leading,
      middle: title,
      trailing: actions,
      centerMiddle: (widget._getEffectiveCenterTitle(themeData))!,
      middleSpacing: widget.titleSpacing,
    );

    // If the toolbar is allocated less than kToolbarHeight make it
    // appear to scroll upwards within its shrinking container.
    Widget appBar = ClipRect(
      child: CustomSingleChildLayout(
        delegate: const _ToolbarContainerLayout(),
        child: IconTheme.merge(
          data: overallIconTheme,
          child: DefaultTextStyle(
            style: (sideStyle)!,
            child: toolbar,
          ),
        ),
      ),
    );
    if (widget.bottom != null) {
      appBar = Column(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        children: <Widget>[
          Flexible(
            child: ConstrainedBox(
              constraints: const BoxConstraints(maxHeight: kToolbarHeight),
              child: appBar,
            ),
          ),
          widget.bottomOpacity == 1.0
              ? widget.bottom!
              : Opacity(
                  opacity:
                      const Interval(0.25, 1.0, curve: Curves.fastOutSlowIn)
                          .transform(widget.bottomOpacity),
                  child: widget.bottom!,
                ),
        ],
      );
    }

    // The padding applies to the toolbar and tabbar, not the flexible space.
    if (widget.primary) {
      appBar = SafeArea(
        top: true,
        child: appBar,
      );
    }

    appBar = Align(
      alignment: Alignment.topCenter,
      child: appBar,
    );

    if (widget.flexibleSpace != null) {
      appBar = Stack(
        fit: StackFit.passthrough,
        children: <Widget>[
          widget.flexibleSpace!,
          appBar,
        ],
      );
    }
    final Brightness brightness = widget.brightness ??
        appBarTheme.brightness ??
        themeData.primaryColorBrightness;
    final SystemUiOverlayStyle overlayStyle = brightness == Brightness.dark
        ? SystemUiOverlayStyle.light
        : SystemUiOverlayStyle.dark;

    return Semantics(
      container: true,
      child: AnnotatedRegion<SystemUiOverlayStyle>(
        value: overlayStyle,
        child: Material(
            color: appBarTheme.color ?? themeData.primaryColor,
            elevation:
                widget.elevation ?? appBarTheme.elevation ?? _defaultElevation,
            shape: widget.shape,
            child: Container(
              decoration: BoxDecoration(gradient: widget.gradient),
              child: Semantics(
                explicitChildNodes: true,
                child: appBar,
              ),
            )),
      ),
    );
  }
}

class _FloatingGradientAppBar extends StatefulWidget {
  const _FloatingGradientAppBar({Key? key, this.child}) : super(key: key);

  final Widget? child;

  @override
  _FloatingGradientAppBarState createState() => _FloatingGradientAppBarState();
}

// A wrapper for the widget created by _SliverGradientAppBarDelegate that starts and
// stops the floating app bar's snap-into-view or snap-out-of-view animation.
class _FloatingGradientAppBarState extends State<_FloatingGradientAppBar> {
  ScrollPosition? _position;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    if (_position != null)
      _position!.isScrollingNotifier.removeListener(_isScrollingListener);
    _position = Scrollable.of(context)?.position;
    if (_position != null)
      _position!.isScrollingNotifier.addListener(_isScrollingListener);
  }

  @override
  void dispose() {
    if (_position != null)
      _position!.isScrollingNotifier.removeListener(_isScrollingListener);
    super.dispose();
  }

  RenderSliverFloatingPersistentHeader? _headerRenderer() {
    return context
        .findAncestorRenderObjectOfType<RenderSliverFloatingPersistentHeader>();
  }

  void _isScrollingListener() {
    // When a scroll stops, then maybe snap the appbar into view.
    // Similarly, when a scroll starts, then maybe stop the snap animation.
    final RenderSliverFloatingPersistentHeader? header = _headerRenderer();
    if (_position!.isScrollingNotifier.value)
      header?.maybeStopSnapAnimation(_position!.userScrollDirection);
    else
      header?.maybeStartSnapAnimation(_position!.userScrollDirection);
  }

  @override
  Widget build(BuildContext context) => widget.child!;
}

class _SliverGradientAppBarDelegate extends SliverPersistentHeaderDelegate {
  _SliverGradientAppBarDelegate({
    required this.leading,
    required this.automaticallyImplyLeading,
    required this.title,
    required this.actions,
    required this.flexibleSpace,
    required this.bottom,
    required this.elevation,
    required this.forceElevated,
    required this.gradient,
    required this.brightness,
    required this.iconTheme,
    required this.actionsIconTheme,
    required this.textTheme,
    required this.primary,
    required this.centerTitle,
    required this.titleSpacing,
    required this.expandedHeight,
    required this.collapsedHeight,
    required this.topPadding,
    required this.floating,
    required this.pinned,
    required this.snapConfiguration,
    required this.shape,
  })   : assert(primary || topPadding == 0.0),
        _bottomHeight = bottom.preferredSize.height;

  final Widget leading;
  final bool automaticallyImplyLeading;
  final Widget title;
  final List<Widget> actions;
  final Widget flexibleSpace;
  final PreferredSizeWidget bottom;
  final double elevation;
  final bool forceElevated;
  final Gradient gradient;
  final Brightness brightness;
  final IconThemeData iconTheme;
  final IconThemeData actionsIconTheme;
  final TextTheme textTheme;
  final bool primary;
  final bool centerTitle;
  final double titleSpacing;
  final double expandedHeight;
  final double collapsedHeight;
  final double topPadding;
  final bool floating;
  final bool pinned;
  final ShapeBorder shape;

  final double _bottomHeight;

  @override
  double get minExtent => collapsedHeight;

  @override
  double get maxExtent => math.max(topPadding + expandedHeight, minExtent);

  @override
  final FloatingHeaderSnapConfiguration snapConfiguration;

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    final double visibleMainHeight = maxExtent - shrinkOffset - topPadding;

    // Truth table for `toolbarOpacity`:
    // pinned | floating | bottom != null || opacity
    // ----------------------------------------------
    //    0   |    0     |        0       ||  fade
    //    0   |    0     |        1       ||  fade
    //    0   |    1     |        0       ||  fade
    //    0   |    1     |        1       ||  fade
    //    1   |    0     |        0       ||  1.0
    //    1   |    0     |        1       ||  1.0
    //    1   |    1     |        0       ||  1.0
    //    1   |    1     |        1       ||  fade
    final double toolbarOpacity = !pinned || (floating)
        ? ((visibleMainHeight - _bottomHeight) / kToolbarHeight).clamp(0.0, 1.0)
        : 1.0;

    final Widget appBar = FlexibleSpaceBar.createSettings(
      minExtent: minExtent,
      maxExtent: maxExtent,
      currentExtent: math.max(minExtent, maxExtent - shrinkOffset),
      toolbarOpacity: toolbarOpacity,
      child: NewGradientAppBar(
        leading: leading,
        automaticallyImplyLeading: automaticallyImplyLeading,
        title: title,
        actions: actions,
        // ignore: unnecessary_null_comparison
        flexibleSpace: (title == null && flexibleSpace != null)
            ? Semantics(child: flexibleSpace, header: true)
            : flexibleSpace,
        bottom: bottom,
        elevation: forceElevated ||
                overlapsContent ||
                (pinned && shrinkOffset > maxExtent - minExtent)
            ? elevation
            : 0.0,
        gradient: gradient,
        brightness: brightness,
        iconTheme: iconTheme,
        actionsIconTheme: actionsIconTheme,
        textTheme: textTheme,
        primary: primary,
        centerTitle: centerTitle,
        titleSpacing: titleSpacing,
        shape: shape,
        toolbarOpacity: toolbarOpacity,
        bottomOpacity:
            pinned ? 1.0 : (visibleMainHeight / _bottomHeight).clamp(0.0, 1.0),
      ),
    );
    return floating ? _FloatingGradientAppBar(child: appBar) : appBar;
  }

  @override
  bool shouldRebuild(covariant _SliverGradientAppBarDelegate oldDelegate) {
    return leading != oldDelegate.leading ||
        automaticallyImplyLeading != oldDelegate.automaticallyImplyLeading ||
        title != oldDelegate.title ||
        actions != oldDelegate.actions ||
        flexibleSpace != oldDelegate.flexibleSpace ||
        bottom != oldDelegate.bottom ||
        _bottomHeight != oldDelegate._bottomHeight ||
        elevation != oldDelegate.elevation ||
        gradient != oldDelegate.gradient ||
        brightness != oldDelegate.brightness ||
        iconTheme != oldDelegate.iconTheme ||
        actionsIconTheme != oldDelegate.actionsIconTheme ||
        textTheme != oldDelegate.textTheme ||
        primary != oldDelegate.primary ||
        centerTitle != oldDelegate.centerTitle ||
        titleSpacing != oldDelegate.titleSpacing ||
        expandedHeight != oldDelegate.expandedHeight ||
        topPadding != oldDelegate.topPadding ||
        pinned != oldDelegate.pinned ||
        floating != oldDelegate.floating ||
        snapConfiguration != oldDelegate.snapConfiguration;
  }

  @override
  String toString() {
    return '${describeIdentity(this)}(topPadding: ${topPadding.toStringAsFixed(1)}, bottomHeight: ${_bottomHeight.toStringAsFixed(1)}, ...)';
  }
}

/// A material design app bar that integrates with a [CustomScrollView].
///
/// An app bar consists of a toolbar and potentially other widgets, such as a
/// [TabBar] and a [FlexibleSpaceBar]. App bars typically expose one or more
/// common actions with [IconButton]s which are optionally followed by a
/// [PopupMenuButton] for less common operations.
///
/// {@youtube 560 315 https://www.youtube.com/watch?v=R9C5KMJKluE}
///
/// Sliver app bars are typically used as the first child of a
/// [CustomScrollView], which lets the app bar integrate with the scroll view so
/// that it can vary in height according to the scroll offset or float above the
/// other content in the scroll view. For a fixed-height app bar at the top of
/// the screen see [NewGradientAppBar], which is used in the [Scaffold.appBar] slot.
///
/// The GradientAppBar displays the toolbar widgets, [leading], [title], and
/// [actions], above the [bottom] (if any). If a [flexibleSpace] widget is
/// specified then it is stacked behind the toolbar and the bottom widget.
///
/// {@tool sample}
///
/// This is an example that could be included in a [CustomScrollView]'s
/// [CustomScrollView.slivers] list:
///
/// ```dart
/// SliverGradientAppBar(
///   expandedHeight: 150.0,
///   flexibleSpace: const FlexibleSpaceBar(
///     title: Text('Available seats'),
///   ),
///   actions: <Widget>[
///     IconButton(
///       icon: const Icon(Icons.add_circle),
///       tooltip: 'Add new entry',
///       onPressed: () { /* ... */ },
///     ),
///   ]
/// )
/// ```
/// {@end-tool}
///
/// ## Animated Examples
///
/// The following animations show how app bars with different configurations
/// behave when a user scrolls up and then down again.
///
/// * App bar with [floating]: false, [pinned]: false, [snap]: false:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.mp4}
///
/// * App bar with [floating]: true, [pinned]: false, [snap]: false:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating.mp4}
///
/// * App bar with [floating]: true, [pinned]: false, [snap]: true:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating_snap.mp4}
///
/// * App bar with [floating]: true, [pinned]: true, [snap]: false:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_pinned_floating.mp4}
///
/// * App bar with [floating]: true, [pinned]: true, [snap]: true:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_pinned_floating_snap.mp4}
///
/// * App bar with [floating]: false, [pinned]: true, [snap]: false:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_pinned.mp4}
///
/// The property [snap] can only be set to true if [floating] is also true.
///
/// See also:
///
///  * [CustomScrollView], which integrates the [SliverNewGradientAppBar] into its
///    scrolling.
///  * [NewGradientAppBar], which is a fixed-height app bar for use in [Scaffold.appBar].
///  * [TabBar], which is typically placed in the [bottom] slot of the [NewGradientAppBar]
///    if the screen has multiple pages arranged in tabs.
///  * [IconButton], which is used with [actions] to show buttons on the app bar.
///  * [PopupMenuButton], to show a popup menu on the app bar, via [actions].
///  * [FlexibleSpaceBar], which is used with [flexibleSpace] when the app bar
///    can expand and collapse.
///  * <https://material.io/design/components/app-bars-top.html>
class SliverNewGradientAppBar extends StatefulWidget {
  /// Creates a material design app bar that can be placed in a [CustomScrollView].
  ///
  /// The arguments [forceElevated], [primary], [floating], [pinned], [snap]
  /// and [automaticallyImplyLeading] must not be null.
  const SliverNewGradientAppBar({
    Key? key,
    this.leading,
    this.automaticallyImplyLeading = true,
    this.title,
    this.actions,
    this.flexibleSpace,
    this.bottom,
    this.elevation,
    this.forceElevated = false,
    this.gradient,
    this.brightness,
    this.iconTheme,
    this.actionsIconTheme,
    this.textTheme,
    this.primary = true,
    this.centerTitle,
    this.titleSpacing = NavigationToolbar.kMiddleSpacing,
    this.expandedHeight,
    this.floating = false,
    this.pinned = false,
    this.snap = false,
    this.shape,
  })  : assert(floating || !snap,
            'The "snap" argument only makes sense for floating app bars.'),
        super(key: key);

  /// A widget to display before the [title].
  ///
  /// If this is null and [automaticallyImplyLeading] is set to true, the [NewGradientAppBar] will
  /// imply an appropriate widget. For example, if the [NewGradientAppBar] is in a [Scaffold]
  /// that also has a [Drawer], the [Scaffold] will fill this widget with an
  /// [IconButton] that opens the drawer. If there's no [Drawer] and the parent
  /// [Navigator] can go back, the [NewGradientAppBar] will use a [BackButton] that calls
  /// [Navigator.maybePop].
  final Widget? leading;

  /// Controls whether we should try to imply the leading widget if null.
  ///
  /// If true and [leading] is null, automatically try to deduce what the leading
  /// widget should be. If false and [leading] is null, leading space is given to [title].
  /// If leading widget is not null, this parameter has no effect.
  final bool automaticallyImplyLeading;

  /// The primary widget displayed in the appbar.
  ///
  /// Typically a [Text] widget containing a description of the current contents
  /// of the app.
  final Widget? title;

  /// Widgets to display after the [title] widget.
  ///
  /// Typically these widgets are [IconButton]s representing common operations.
  /// For less common operations, consider using a [PopupMenuButton] as the
  /// last action.
  ///
  /// {@tool sample}
  ///
  /// ```dart
  /// Scaffold(
  ///   body: CustomScrollView(
  ///     primary: true,
  ///     slivers: <Widget>[
  ///       SliverGradientAppBar(
  ///         title: Text('Hello World'),
  ///         actions: <Widget>[
  ///           IconButton(
  ///             icon: Icon(Icons.shopping_cart),
  ///             tooltip: 'Open shopping cart',
  ///             onPressed: () {
  ///               // handle the press
  ///             },
  ///           ),
  ///         ],
  ///       ),
  ///       // ...rest of body...
  ///     ],
  ///   ),
  /// )
  /// ```
  /// {@end-tool}
  final List<Widget>? actions;

  /// This widget is stacked behind the toolbar and the tab bar. It's height will
  /// be the same as the app bar's overall height.
  ///
  /// Typically a [FlexibleSpaceBar]. See [FlexibleSpaceBar] for details.
  final Widget? flexibleSpace;

  /// This widget appears across the bottom of the appbar.
  ///
  /// Typically a [TabBar]. Only widgets that implement [PreferredSizeWidget] can
  /// be used at the bottom of an app bar.
  ///
  /// See also:
  ///
  ///  * [PreferredSize], which can be used to give an arbitrary widget a preferred size.
  final PreferredSizeWidget? bottom;

  /// The z-coordinate at which to place this app bar when it is above other
  /// content. This controls the size of the shadow below the app bar.
  ///
  /// If this property is null, then [ThemeData.appBarTheme.elevation] is used,
  /// if that is also null, the default value is 4, the appropriate elevation
  /// for app bars.
  ///
  /// If [forceElevated] is false, the elevation is ignored when the app bar has
  /// no content underneath it. For example, if the app bar is [pinned] but no
  /// content is scrolled under it, or if it scrolls with the content, then no
  /// shadow is drawn, regardless of the value of [elevation].
  final double? elevation;

  /// Whether to show the shadow appropriate for the [elevation] even if the
  /// content is not scrolled under the [NewGradientAppBar].
  ///
  /// Defaults to false, meaning that the [elevation] is only applied when the
  /// [NewGradientAppBar] is being displayed over content that is scrolled under it.
  ///
  /// When set to true, the [elevation] is applied regardless.
  ///
  /// Ignored when [elevation] is zero.
  final bool forceElevated;

  /// The gradient displayed at the appbar.
  ///
  /// If this property is null, then [ThemeData.appBarTheme.color] is used,
  /// if that is also null, then [ThemeData.primaryColor] is used.
  final Gradient? gradient;

  /// The brightness of the app bar's material. Typically this is set along
  /// with [backgroundColor], [iconTheme], [textTheme].
  ///
  /// If this property is null, then [ThemeData.appBarTheme.brightness] is used,
  /// if that is also null, then [ThemeData.primaryColorBrightness] is used.
  final Brightness? brightness;

  /// The color, opacity, and size to use for app bar icons. Typically this
  /// is set along with [backgroundColor], [brightness], [textTheme].
  ///
  /// If this property is null, then [ThemeData.appBarTheme.iconTheme] is used,
  /// if that is also null, then [ThemeData.primaryIconTheme] is used.
  final IconThemeData? iconTheme;

  /// The color, opacity, and size to use for trailing app bar icons. This
  /// should only be used when the trailing icons should be themed differently
  /// than the leading icons.
  ///
  /// If this property is null, then [ThemeData.appBarTheme.actionsIconTheme] is
  /// used, if that is also null, then this falls back to [iconTheme].
  final IconThemeData? actionsIconTheme;

  /// The typographic styles to use for text in the app bar. Typically this is
  /// set along with [brightness] [backgroundColor], [iconTheme].
  ///
  /// If this property is null, then [ThemeData.appBarTheme.textTheme] is used,
  /// if that is also null, then [ThemeData.primaryTextTheme] is used.
  final TextTheme? textTheme;

  /// Whether this app bar is being displayed at the top of the screen.
  ///
  /// If this is true, the top padding specified by the [MediaQuery] will be
  /// added to the top of the toolbar.
  final bool primary;

  /// Whether the title should be centered.
  ///
  /// Defaults to being adapted to the current [TargetPlatform].
  final bool? centerTitle;

  /// The spacing around [title] content on the horizontal axis. This spacing is
  /// applied even if there is no [leading] content or [actions]. If you want
  /// [title] to take all the space available, set this value to 0.0.
  ///
  /// Defaults to [NavigationToolbar.kMiddleSpacing].
  final double titleSpacing;

  /// The size of the app bar when it is fully expanded.
  ///
  /// By default, the total height of the toolbar and the bottom widget (if
  /// any). If a [flexibleSpace] widget is specified this height should be big
  /// enough to accommodate whatever that widget contains.
  ///
  /// This does not include the status bar height (which will be automatically
  /// included if [primary] is true).
  final double? expandedHeight;

  /// Whether the app bar should become visible as soon as the user scrolls
  /// towards the app bar.
  ///
  /// Otherwise, the user will need to scroll near the top of the scroll view to
  /// reveal the app bar.
  ///
  /// If [snap] is true then a scroll that exposes the app bar will trigger an
  /// animation that slides the entire app bar into view. Similarly if a scroll
  /// dismisses the app bar, the animation will slide it completely out of view.
  ///
  /// ## Animated Examples
  ///
  /// The following animations show how the app bar changes its scrolling
  /// behavior based on the value of this property.
  ///
  /// * App bar with [floating] set to false:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.mp4}
  /// * App bar with [floating] set to true:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating.mp4}
  ///
  /// See also:
  ///
  ///  * [SliverNewGradientAppBar] for more animated examples of how this property changes the
  ///    behavior of the app bar in combination with [pinned] and [snap].
  final bool floating;

  /// Whether the app bar should remain visible at the start of the scroll view.
  ///
  /// The app bar can still expand and contract as the user scrolls, but it will
  /// remain visible rather than being scrolled out of view.
  ///
  /// ## Animated Examples
  ///
  /// The following animations show how the app bar changes its scrolling
  /// behavior based on the value of this property.
  ///
  /// * App bar with [pinned] set to false:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.mp4}
  /// * App bar with [pinned] set to true:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_pinned.mp4}
  ///
  /// See also:
  ///
  ///  * [SliverNewGradientAppBar] for more animated examples of how this property changes the
  ///    behavior of the app bar in combination with [floating].
  final bool pinned;

  /// The material's shape as well its shadow.
  ///
  /// A shadow is only displayed if the [elevation] is greater than
  /// zero.
  final ShapeBorder? shape;

  /// If [snap] and [floating] are true then the floating app bar will "snap"
  /// into view.
  ///
  /// If [snap] is true then a scroll that exposes the floating app bar will
  /// trigger an animation that slides the entire app bar into view. Similarly if
  /// a scroll dismisses the app bar, the animation will slide the app bar
  /// completely out of view.
  ///
  /// Snapping only applies when the app bar is floating, not when the appbar
  /// appears at the top of its scroll view.
  ///
  /// ## Animated Examples
  ///
  /// The following animations show how the app bar changes its scrolling
  /// behavior based on the value of this property.
  ///
  /// * App bar with [snap] set to false:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating.mp4}
  /// * App bar with [snap] set to true:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating_snap.mp4}
  ///
  /// See also:
  ///
  ///  * [SliverNewGradientAppBar] for more animated examples of how this property changes the
  ///    behavior of the app bar in combination with [pinned] and [floating].
  final bool snap;

  @override
  _SliverNewGradientAppBarState createState() =>
      _SliverNewGradientAppBarState();
}

// This class is only Stateful because it owns the TickerProvider used
// by the floating appbar snap animation (via FloatingHeaderSnapConfiguration).
class _SliverNewGradientAppBarState extends State<SliverNewGradientAppBar>
    with TickerProviderStateMixin {
  FloatingHeaderSnapConfiguration? _snapConfiguration;

  void _updateSnapConfiguration() {
    if (widget.snap && widget.floating) {
      _snapConfiguration = FloatingHeaderSnapConfiguration(
        curve: Curves.easeOut,
        duration: const Duration(milliseconds: 200),
      );
    } else {
      _snapConfiguration = null;
    }
  }

  @override
  void initState() {
    super.initState();
    _updateSnapConfiguration();
  }

  @override
  void didUpdateWidget(SliverNewGradientAppBar oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.snap != oldWidget.snap || widget.floating != oldWidget.floating)
      _updateSnapConfiguration();
  }

  @override
  Widget build(BuildContext context) {
    assert(!widget.primary || debugCheckHasMediaQuery(context));
    final double topPadding =
        widget.primary ? MediaQuery.of(context).padding.top : 0.0;
    final double collapsedHeight = ((widget.pinned && widget.floating)
        ? widget.bottom!.preferredSize.height + topPadding
        : null)!;

    return MediaQuery.removePadding(
      context: context,
      removeBottom: true,
      child: SliverPersistentHeader(
        floating: widget.floating,
        pinned: widget.pinned,
        delegate: _SliverGradientAppBarDelegate(
          leading: (widget.leading)!,
          automaticallyImplyLeading: widget.automaticallyImplyLeading,
          title: widget.title!,
          actions: widget.actions!,
          flexibleSpace: widget.flexibleSpace!,
          bottom: widget.bottom!,
          elevation: widget.elevation!,
          forceElevated: widget.forceElevated,
          gradient: widget.gradient!,
          brightness: widget.brightness!,
          iconTheme: widget.iconTheme!,
          actionsIconTheme: widget.actionsIconTheme!,
          textTheme: widget.textTheme!,
          primary: widget.primary,
          centerTitle: widget.centerTitle!,
          titleSpacing: widget.titleSpacing,
          expandedHeight: widget.expandedHeight!,
          collapsedHeight: collapsedHeight,
          topPadding: topPadding,
          floating: widget.floating,
          pinned: widget.pinned,
          shape: widget.shape!,
          snapConfiguration: (_snapConfiguration)!,
        ),
      ),
    );
  }
}
